Natural populations of Drosophila mercatorum have all the properties common to other species of Drosophila but has one, parthenogenesis, which allows one to experimentally control factors which affect genetic variation. This species was used as the biological design to estimate the size of the genetic units whose fitnesses combine in a mathematically simple additive or multiplicative fashion to explain genotypic fitness. Deviations from random expectations of genotype frequencies in parthenogenetic progeny of heterozygous females could be attributed to selection because no factors other than meiosis affect the observations. The data collected gave evidence for true coadaptation involving non-additive (non-multiplicative) interactions between nonalleles. The unit of selection was found to be a function of the genetic state of the background. The greater the perturbation of the coadapted genotype by meiosis the larger the unit of genetic material which behaves as an additive (multiplicative) unit. Selective neutrality of allelic variation may be an artifact of our failure to measure the proper genetic unit.